docs: def _relevant_connections(self, subsystem):
          """Identify connections that “matter” to this concept.
  
          For a |MIC|, the important connections are those which connect the
          purview to the mechanism; for a |MIE| they are the connections from the
          mechanism to the purview.
  
          Returns an |N x N| matrix, where `N` is the number of nodes in this
          corresponding subsystem, that identifies connections that “matter” to
          this MICE:
  
          ``direction == Direction.CAUSE``:
              ``relevant_connections[i,j]`` is ``1`` if node ``i`` is in the
              cause purview and node ``j`` is in the mechanism (and ``0``
              otherwise).
  
          ``direction == Direction.EFFECT``:
              ``relevant_connections[i,j]`` is ``1`` if node ``i`` is in the
              mechanism and node ``j`` is in the effect purview (and ``0``
              otherwise).
  
          Args:
              subsystem (Subsystem): The |Subsystem| of this MICE.
  
          Returns:
              np.ndarray: A |N x N| matrix of connections, where |N| is the size
              of the network.
  
          Raises:
              ValueError: If ``direction`` is invalid.
          """
          _from, to = self.direction.order(self.mechanism, self.purview)
          return connectivity.relevant_connections(subsystem.network.size,
                                                   _from, to)